1. Field of the Invention
The present invention relates generally to an automatic automotive transmission and more specifically to an arrangement therefor which attenuates shift shock over a wide range of engine load and speed.
2. Description of the Prior Art
FIG. 1 shows a portion of a prior art transmission disclosed in detail in SERVICE MANUAL Nissan Automatic Transmission Model 3N71B. This section of the transmission as shown, includes a front clutch 1, a band brake 2 and a 2-3 shift valve 3 which are interconnected as shown. The disclosure of the above mentioned service manual is hereby incorporated by reference thereto.
The band brake 2 is arranged to have a servo 4 which includes an apply chamber 6 and a release chamber 8. The stepped piston 10 which is disposed between the opposed chambers 6, 8 is arranged so that the surface area exposed to the release chamber 8 is greater than that exposed to the apply chamber 6 so that even if both chambers are applied with pressures of equal magnitude (eg. both chambers are supplied with line pressure) the brake will assume a released condition upon pressurization of the release chamber 8.
The 2-3 shift valve 3 includes a spool 12 which when in the downshift position (upper half section) connects port 14 with a drain 16 while in the upshift position (lower half section) connects port 14 with a port 18 which is supplied with line pressure. Hence, while the transmission is conditioned to provide 2nd speed and shift valve 3 is in its downshift position, chamber 20 of the forward clutch servo 22 and the release chamber 8 of the band brake servo 4 are depressurized. This induces the front clutch 1 to assume a released condition while the band brake 2 is applied due pressure being applied only to the apply chamber 6.
Upon the shift valve 3 assuming its upshift position under the influence of governor (P.sub.G) and throttle (P.sub.N) pressures, the transmission is condition to assume 3rd speed by the application of line pressure (P.sub.L) to chambers 20 and 8 which applies the front clutch 1 and releases the band brake 2.
In this arrangement a flow restricting orifice 24 is interposed between port 14 and chambers 8 and 20. In actual fact, two orifices are disposed in the conduits at locations indicated schematically by the phantom boxes. However, for simplicity of illustration only one orifice is shown and is illustrated in a position wherein the effect of the above mentioned two is provided.
This arrangement however, has suffered from the drawbacks that as shown in FIG. 2 when the transmission upshifts at low engine throttle opening a shift shock is encountered. The reason for this is that when the engine throttle opening is small the band brake 2 tends to be released relatively rapidly as compared with the engagement of the front clutch 1 so that at time t1 the transmission assumes a condition in which neither of 2nd or 3rd gears are established and the engine tends to race as indicated by .DELTA.Ne due to the sudden `quasi neutral` condition which occurs. This induces a peak in the torque applied to the output shaft of the transmission as indicated by .DELTA.T1 and hence results in the generation of shift shock upon a 2-3 upshift.
On the other hand, when a 2-3 upshift occurs at a high throttle opening, the release of the band brake 2 tends to be delayed with respect to the engagement of front clutch 1 so that at time t2 a negative torque (.DELTA.T2) tends to be applied to the output shaft (not shown) due to slight locking of the transmission as shown in FIG. 3. This induces a subsequent sharp peak (.DELTA.T3) in the output shaft torque which causes shift shock.
FIG. 4 shows a second example of prior art. This arrangement is disclosed in detail in SERVICE MANUAL Nissan Automatic Transmission Model RN 3F01A. This arrangement differs from the one shown in FIG. 1 in that only the communication between port 14 and the release chamber 8 of the band brake 2 is restricted by a flow restriction 26. The reason for this provision is to enable the slow engagement of the band brake 2 with respect to the release of the front clutch 1 in an effort to smooth the shift by enabling the engine to race slightly in a manner which brings the engine speed up to a level which matches that which will result after the shift is completed.
However, this arrangement has encountered the drawback that during downshifting from second to third gear at low vehicle speed, due to the low engine speed the centrifugal force acting on front cluch 1 is correspondingly low, whereby the force holding the hydraulic fluid in chamber 20 tends to be very low and the depressurization thereof essentially instantaneous thus instantly reducing the torque transmission thereby. However, at this time the engagement characteristics of the band brake 2 are not effected by centrifugal force (engine speed) and are determined only by orifice 26.
Under these conditions, the engine tends to race more than required over a period to and induce an engine speed peak as indicated by .DELTA.Ne in FIG. 5. As a result, a shift shock generating peak (.DELTA.T1) in the output shaft torque tends to occur.
On the other hand, in the event that a 3-2 downshift occurs a high vehicle speed the release charactertics of the front clutch 1 are influenced by the increased amount of centrifugal force acting on the device whereby the band brake 2 tends to engage more rapidly (with respect to the release of the front clutch) and induces the situation wherein a slight lock-up of the transmission occurs. This causes the output shaft torque to drop sharply and even become negative as shown by .DELTA.T2 in FIG. 6 and induce shift shock.